1. Field of the Invention
The invention relates to fixed bed reactors for the reaction of gaseous starting materials or mixtures of starting materials which are characterised in that the starting materials or mixtures of starting materials flow through a catalyst bed, which is designed as a sheet, approximately or completely perpendicularly to these surfaces.
In this case it is possible to allow the function which describes the temperature and the starting material load in relation to position in the catalyst bed to have as flat a course as possible, as a result of which partial overloads and--especially in the case of highly exothermic reactions--temperature peaks are avoided.
These characteristics of the reactors according to the invention result in optimal selectivities and operating lives of the catalyst.
The invention further relates to the use of such fixed bed reactors for the reaction of gaseous starting materials or mixtures of starting materials.
The catalytic reaction of gases on stationary catalysts can easily be carried out as a continuous process and has the attractive convenience that the product does not generally have to be separated from auxiliaries, such as solvents or catalysts. Accordingly, preference is being given in the chemical industry to production of chemicals having medium and large production quantities in this way.
Many organic compounds are significantly more stable in the gaseous state of matter than in the liquid state, since, because of the low spatial concentration, bimolecular and higher molecular side reactions are repressed, so that, for example, highly exothermic reactions can be carried out at a higher temperature level than in the liquid state, which is in turn advantageous for coupled thermal energy recovery.
Other reactions, for example, endothermic cleavage reactions, require particularly high temperatures and can virtually only be carried out in the gas phase, in particular in the case of sensitive starting materials and products.
2. Description of the Related Art
A multiplicity of reactor types are described in the literature (Ullmanns, Enzyklopadie der technischen Chemie [Encyclopaedia of Industrial Chemistry], 4th edition, Volume 3, pp. 468-469; Kirk-Othmer, Encyclopedia of Chemical Technology, Volume 19 (1982), pp. 880-914). In this case, a distinction is made between reactors having a controlled temperature profile and those having an uncontrolled temperature profile.
Reactors having an uncontrolled temperature profile are composed of a dumped catalyst packing or of a catalyst bed made of catalytically active shaped bodies in a vessel, insulated to a greater or lesser extent, and of simple geometry.
The enthalpy of reaction in such reactors is passed on to the product mixture; therefore these types of construction are termed adiabatic. The main attraction of these reactors is their simple, and thus cheap and easily-maintained, construction. However, when such adiabatic reactors having an uncontrolled temperature profile are used, account should be taken of the fact that chemical changes have an exponential character both with respect to kinetic and thermodynamic aspects and thus react very sensitively to temperature changes. This means that many reactions proceed with sufficient selectivity and at a sufficient rate in a narrow temperature range. The catalyst itself can also prove to be temperature-sensitive.
Since the majority of chemical reactions have a significant heat of reaction in relation to the thermal capacity of the systems, only in very few cases can the removal or supply of thermal energy to maintain the required temperature range be dispensed with (controlled temperature profile).
In the case of the reactors having a controlled temperature profile, a distinction is made between the continuous and the stepwise thermostatting, that is heat removal or heat supply. The stepwise thermostatting in principle breaks down an adiabatic reactor into part-sections, after leaving which the gas is brought into thermal interaction with a heat transport medium or receives an admixture of fresh starting material mixture at an appropriate temperature (Ullman, loc. cit., p. 473). In this type of reactor, the reaction should not be too thermally demanding since otherwise a very large number of stages is required in order to maintain the temperature limits. On the other hand, highly complicated reactor structures have been developed especially for the synthesis of ammonia and methanol for the optimal housing and optimal operation of a relatively large number of catalyst bed sections and heat exchangers in a high pressure chamber (EP 297,474, EP 333,757 and literature cited therein).
For reactions having extremely high heat of reaction and catalysts or reactions having extremely sensitive temperature behaviour therefore, continuous thermostatting is to be installed. In this case, the catalyst can be housed either between the tubes or in the tubes of a heat exchanger (Linde reactor according to German Offenlegungsschrift 3,414,717 or tube bundle reactor according to Chemie-Ingenieur-Technik 51 (1979), p. 257-265). Reactors of this type having tube diameters of one to several centimeters and tube lengths of 2-20 m have been prior art for a long time. Despite the constant heat flow in a radial direction, in strongly exothermic reactions a hot-spot forms in the reactor tubes, which is responsible for losses in selectivity or catalyst damage.
It has already been described to diminish the hot-spot by diluting the catalyst at the beginning of packing with inert material or by the reactor tubes at the inlet end having a lower diameter than at the outlet end (German Offenlegungsschrift 2,929,300; GB 2,132,111). It has furthermore been described to alleviate the lack of uniform thermostatting by shortening the tubes of the tube bundle reactor to 1-30 cm, preferably 5-20 cm (German Offenlegungsschrift 3,612,213, EP 244,632). The tubes of such a short-tube reactor have internal diameters of 0.5-3 cm, preferably 1-2 cm. The advantages of this short-tube reactor are described as follows:
Large specific cooling surface area and uniform streaming of the heat transport medium to the tubes from the side lead to a defined temperature distribution in the reactor tube, uniform lateral streaming of the heat transport medium to all tubes is intended to lead to a compact construction with maximum heat transfer; even in the case of fine-grain catalyst a low pressure drop is said to result in the tubes.
In EP 244,632 mentioned, in addition an association is made between tube length and residence time without considering this in more detail; the short tube with short residence time and good heat dissipation is described as expedient for highly exothermic reactions having high energy of activation and thermally unstable reactants and products. However, in this case, no attention is paid to the number of tubes which a short tube bundle reactor must accommodate, the tube length of which is shorter than that of a current reactor by a factor of 50-500, in order to make possible industrially relevant product flow rates. In order to substitute a reactor not infrequently to be encountered in industry having 10,000 tubes of conventional length, these would have to be 500,000 to 5 million short tubes built into the short tube bundle reactor. Just as little attention is paid to the uniform flow through this giant number of tubes or the manner in which this is to be ensured or the manner in which the tubes are to be filled. EP 244 632 mentioned is exclusively concerned with the residence time and the uniform thermostatting, since according to known teaching the superficial loading of a catalyst bed (1 of starting material mixture per m.sup.2 of external surface area of the catalyst bed and hour) is generally not a particularly characteristic parameter for the properties of a heterogeneous catalysed gas-phase reaction.